Lubrication and cooling of mechanical equipment such as gearboxes, bearings or combustion engines is typically obtained by either of the following principles:
An oil pump with constant geometric volume is driven by a constant or variable speed electric motor, more recently also by frequency controlled motors allowing a continuous variation of the pump speed and thereby the oil flow. This arrangement allows continuous adjustment of the flow to the momentary needs by an external controller as long as electrical power is available. In case of loss of this external source of energy, the oil flow ceases, and a safe run-down of the equipment cannot be granted.
An oil pump with constant geometric volume is driven by a shaft of the equipment, for example a power-take-off (PTO) from a gearbox. The oil flow is hence directly dependent on the speed of the drive shaft, and cannot be adjusted to the momentary needs. This becomes a particular disadvantage in applications where the speed of the PTO-shaft varies. Obtaining sufficient oil supply at the lowest operating speeds may require selection of quite large pumps, which will then supply too much oil in the upper speed range. The surplus oil needs to be wasted through bypasses, which increases the system's complexity.
Additionally, excessive circulation deteriorates the oil, causes premature aging, and will typically require increased oil volumes. Compared to electrical driven pumps, such shaft driven pumps allow a safe run-down also in case the external power supply collapses. The efficiency will typically be higher, as no additional power transformation is required.
A common solution combining the advantages of shaft-driven and electrical pumps is installing two independent systems where the shaft-driven pump cares for sufficient supply when no external power is available, and where the electrical pump or both, i.e, also the shaft-driven pumps in parallel provide the oil supply in regular operation. Two independent systems are more costly and more complex systems
Shaft driven pumps, where the geometric volume of the oil pump is varied, is a third possibility of obtaining variable oil flow independent of the speed of the equipment. This technology is typically used in automotive systems and hydraulic applications, but has technical limitations for large oil flow, or for fluids with high viscosity as typically used in industrial applications, due to the limited suction capacity of those pump designs.